1. Field of the Invention
This invention relates to a rotary kiln for calcining loose bulk materials by application of heat during rotation.
2. Description of the Prior Art
The conventional rotary kiln comprises a simple cylinder installed at an incline. Raw material feed to be calcined by the application of heat is charged into the kiln from an inlet at the elevated end, the feed is calcined by applying heat while the cylinder is rotated, and the calcined product is discharged from an outlet at the lower end. To provide a source of heat for effecting calcination, combustion gases obtained by burning a combustible material near the outlet of the cylinder are introduced into the kiln from the outlet and move countercurrent to the flow of feed along the cylinder. Though kilns of this type are advantageous in that they are capable of treating raw materials of many different kinds and of a wide variety of particle diameters, the raw material is allowed to come into direct contact with the ash-containing combustion gases. As a result, the ash content mixes with the raw material and detracts from the quality of the final product, especially in cases where coal having a high ash content is used as the combustible material. In particular, using a waste having a high content of unburned inorganic substances as the combustible material can cause so much ash to mix with the calcined product as to render the product unusable.
Moreover, a rotary kiln of simple cylindrical construction presents a small heat receiving surface area per unit length of the kiln in comparison with the amount of heat given off by the kiln. This makes it necessary for the rotary kiln to be of considerable length in order that a sufficient amount of thermal energy derived from the flame and combustion gases may be transmitted to the raw material. However, a disadvantage of rotary kilns of greater length is an increase in the amount of heat given off by the outer surface of the kiln, thus making it impossible to improve heating efficiency.
Several configurations for burning the combustible material are available. These include a fire grate arrangement in which the combustible material is burned on a grate, a fluidized bed set-up in which the raw material feed and combustible material are mixed together and burned while being made to flow, and an arrangement in which a burner is used. The fire grate allows finely divided combustible materials to fall through the interstices of the grate and is therefore ill suited for such materials. The fluidized bed and burner arrangements can only be applied to granulated coal and finely divided coal, respectively, and therefore place a limitation upon the types of combustible material that can be used.